A plurality of independent cells having a regular octagonal or circular cross section are arranged in the form of a lattice. Cross-sectionally square independent cells are substituted for two octagonal or circular cells in the central portion of the lattice arrangement. These two square independent cells are so formed that each side of the cells has the largest possible length when the cells are placed among the adjacent cells having a regular octagonal or circular cross section. Spacer support rods serving also as water rods are inserted into the square cells in such a fuel spacer, which consists of a combination of cells having a regular octagonal or circular cross section and cells having a square cross section as mentioned above, to form a fuel assembly.